Room temperature hydrogen sensor

ABSTRACT

A sensor for selectively determining the presence and measuring the amount of hydrogen in the vicinity of the sensor. The sensor comprises a MEMS device coated with a nanostructured thin film of indium oxide doped tin oxide with an over layer of nanostructured barium cerate with platinum catalyst nanoparticles. Initial exposure to a UV light source, at room temperature, causes burning of organic residues present on the sensor surface and provides a clean surface for sensing hydrogen at room temperature. A giant room temperature hydrogen sensitivity is observed after making the UV source off. The hydrogen sensor of the invention can be usefully employed for the detection of hydrogen in an environment susceptible to the incursion or generation of hydrogen and may be conveniently used at room temperature.

This invention relates to a room temperature hydrogen sensor, and morespecifically to a room temperature hydrogen sensor, which uses microelectro-mechanical systems (MEMS) device, an ultraviolet (UV) radiation,and the nanocstructured indium oxide (In₂O₃) doped tin-oxide (SnO₂) thinfilm as a hydrogen sensor with the nanostrucured inorganic thin film Hand metal catalyst as hydrogen separation membrane overlayers toselectively detect hydrogen gas in the environment at room temperature.

BACKGROUND AND PRIOR ART

Hydrogen is a flammable and explosive gas with a wide variety ofindustrial and scientific uses. It is axiomatic that handling hydrogenrequires utilization of robust safety devices since hydrogen is a highlyflammable gas at concentrations in air as low as 4% by volume.Well-known industrial uses of hydrogen include the production of basicstaple products of the chemical industry such as ammonia and fertilizersderived there from. Other uses include basic alcohols, hydrogenchloride, reduction of ores for the manufacturing of metals, refinery ofoil for the manufacturing of petroleum, and the hydrogenation ofvegetable oils for margarine and related industries.

Hydrogen is also widely used for space flight applications, for example,hydrogen is used as a component of hydrogen-oxygen blends used invehicular propulsion systems. Hydrogen gas is also used in theprocessing of rocket fuel in the aerospace industry. The combustiblenature of hydrogen however, makes its detection vitally important.

Hydrogen is also utilized in a variety of metal forming andmicroelectronic processing steps which are often of extreme importancein device fabrication and metal interconnect processing of multi-levelmicroelectronic devices.

The increase in oil prices also increased the emphasis on the use offuel cells, which require hydrogen as a fuel in various stationary andmobile applications, for instance, in fuel cells of automobiles.

In these and other applications, hydrogen sensors are employed tomonitor the environment around which hydrogen is utilized, to ensure theefficiency, safety and operational integrity of process systems. Forsuch purposes, a number of hydrogen sensors and complex detectionmethods have been developed and are in common use. About one-half of allthe sensors used to measure hazardous gases measure hydrogen. The bulkof these systems utilize as the detector element a Group VIIIB metalelement, for example, Ni, Pd, or Pt, heated to catalytically oxidize thehydrogen, with the resulting change in heat load being the measuredparameter for determination of the presence of hydrogen.

A variety of these commercially available hydrogen sensors are based onmeasuring an electrical characteristic across a sensor element and atleast four major categories of sensors and associated methods have beenidentified.

One type of hydrogen sensor is the “catalytic combustible” or “hot wire”sensor (CC sensor) mentioned in the U.S. Pat. No. 6,006,582 to Bhandari,et. al. The CC sensor comprises two specially arranged beads of acatalytic metal or alloy, such as platinum-iridium wire heated to600-800 degrees Celsius. One bead is coated with a reactive catalyst. Inthe presence of a flammable gas, the heat of oxidation raises thetemperature of the bead and alters the electrical resistancecharacteristics of the measuring circuit. This resistance change isrelated to the concentration of all flammable gases, including hydrogen,in the vicinity of the sensor.

Sensors of such “hot wire” type have cross-sensitivity to other easilyoxidized materials, such as alcohols and hydrocarbons. Such easilyoxidized materials are common components of gases in asemiconductor-manufacturing environment, and often result in thefrequent occurrence of false alarms.

Current hot wire sensors require an oxidation reaction for operation,such sensors are unable to detect hydrogen when it is present in inertgas streams or environments which are not of a character to support anoxidative reaction. This is a deficiency of such hot wire sensors andlimits their applicability and utility.

The CC sensor has drawbacks. In oxygen deficient environments or abovean upper explosive limit, the oxidation process is quenched causingdifficulties in measuring. In addition, since the CC sensor is basedupon oxidation, and all hydrocarbons have the same response as hydrogen,this makes it difficult to detect hydrogen in environments which alsocontain hydrocarbons. Further, the CC sensor element is easilycontaminated by halogenated hydrocarbons and is susceptible to poisoningby silicones, lead and phosphorous.

Another commonly used hydrogen sensor is a non-porous metal oxide (MO)sensor. The MO sensor element comprises a non-porous metal oxide (suchas zirconium dioxide or tin dioxide) sandwiched between two porous metalelectrodes. Such electrodes are typically made of platinum. Oneelectrode is exposed to the reference gas, usually air, and the otherelectrode is exposed to the test gas being detected.

Mobile ions diffuse to both surfaces of the oxide where they may beeliminated by reaction with adsorbed species. In the absence of gasspecies which can be oxidized (such as, for instance, carbon monoxide orhydrogen), the electrochemical potential of the sensor may be determinedby the Nernst equation and is proportional to the partial pressure ofoxygen in the test gas only. In order to achieve sensitivity to hydrogenwith this device, the platinum electrode is co-deposited with gold.Since gold is a substantially less efficient donor of electrons thanplatinum, oxidation rates are reduced, equilibrium conditions are notachieved and the sensor becomes sensitive to the composition of the testgas. The electrochemical potential which develops becomes“non-Nernstian”, and is a complicated function of the kinetics and masstransfer associated with all species reacting at the electrode.

Like the CC sensor, the MO sensor has serious disadvantages. The sensoris not hydrogen-specific and all oxidizable gases in the test gascontribute to the sensor signal. The response is relatively slow and itcan take up to 20 seconds to reach 50% of maximum signal when exposed to1% hydrogen in air at flows below 200 standard cubic centimeters perminute (sccm); the recovery time is even slower taking up to 5 minutesto reach 50% of maximum signal when exposed to less than 200 sccm ofair. Finally, in order to achieve even these orders of response time,the device must be operated at temperatures above 350 degrees Celsius.Operating at such temperatures, is potentially unsafe and may causeignition and/or explosion.

Another class of sensors includes metal-insulator semiconductor (MIS) ormetal-oxide-semiconductor (MOS) capacitors and field effect transistors,as well as palladium-gated diodes. In general however, these sensors arelimited to detecting low concentrations of hydrogen.

Yet another type of sensor is the metal oxide-semiconductor (MOS) sensorwhich is also known and is mentioned, for instance, in the U.S. Pat. No.6,006,582 to Bhandari, et. al. The MOS sensor element comprises anoxide, typically of iron, zinc, or tin, or a mixture thereof, and isheated to a temperature of about 150 degrees Celsius to about 350degrees Celsius. Bhandari et. al. reported that oxygen absorbs on thesurface of the sensor element to create an equilibrium concentration ofoxide ions in the surface layers.

The original resistance of the MOS sensor is first measured. Whencertain compounds, such as, for instance, CO, or hydrocarbons come incontact with the sensor, they are adsorbed on the surface of the MOSelement. This absorption shifts the oxygen equilibrium, causing adetectable increase in conductivity of the MOS material.

MOS hydrogen sensors have a number of operational deficiencies and are,therefore, unsatisfactory in many respects. They require frequentcalibration and their response times are too long (up to 3-5 minutes).Bhandari et. al. noted that the MOS sensors are unsafe and can causeignition and explosion, and are susceptible to being poisoned withhalogenated vapors. Like the CC and the MO sensors discussed above, theyare not hydrogen specific. All volatile organic compounds as well asgases containing hydrogen will react with the sensor materials in thesensing elements of these detectors, thereby providing false readings.

Still yet another sensor is the catalytic gate (CG) sensor, the simplestembodiment of which is a MOS structure, where the metal is usuallyplatinum or palladium deposited on an insulator, such as silicondioxide. Hydrogen dissociates on platinum or palladium and subsequentlydiffuses into the bulk of the metal. Hydrogen atoms which arrive at themetal-insulator interface, form a dipole layer, polarizing the interfaceand consequently changing its electrical characteristics. The CG sensoralso has serious drawbacks, particularly slow response time when thesurface is contaminated. The surface of platinum or palladium is verymuch susceptible to contamination and poisoning.

There exists no known prior art teaching of a hydrogen-specific sensor,which quickly responds only to hydrogen gas at room temperature andwhich is not susceptible to poisoning. Yet, as discussed above, suchsensor is highly desirable and the need for such sensor, which is alsolow cost, lightweight and of a miniature size, is acute.

The present invention discloses such a sensor. It therefore is an objectof the present invention to provide an improved hydrogen selectivesensor and hydrogen sensing methodology overcoming the aforementioneddeficiencies of the previously known hydrogen detectors.

Because hydrogen is used in such a wide variety of environments, it isdesirable to have a sensor that will be reproducible and specific tohydrogen, even with varying concentration of background gases such asoxygen, water and other contaminants.

It is also desirable to have a solid state sensor, operating at roomtemperature, that has no moving parts, has a response time on the orderof seconds, would operate with minimum power consumption, does notrequire frequent calibration, and could be used in a hand-held portableinstrument.

SUMMARY OF THE INVENTION

It therefore is one object of the present invention to provide animproved hydrogen sensor.

It is another object of the invention to provide a hydrogen sensor thatdetects only the presence of hydrogen at room temperature.

It is another object of the invention to provide a hydrogen sensor thatsenses the presence of hydrogen, at room temperature, in a reproducibleand hydrogen-specific manner.

It is another object of the invention to provide a hydrogen sensor thatsenses the presence of hydrogen, at room temperature, in a reproducibleand hydrogen-specific manner, even with varying concentrations ofbackground gases such as oxygen, water and other contaminants.

It is another object of the invention to provide a hydrogen sensor thatsenses the presence of hydrogen, at room temperature, in a reproducibleand hydrogen-specific manner, even with varying concentrations ofbackground gases such as oxygen, water and other contaminants and isself-cleaning in nature.

One aspect of the present invention embodiments relates to a method ofdetecting hydrogen in an environment comprising:

providing a MEMS based hydrogen sensor device comprising a substrate, ahydrogen-sensitive thin film with the hydrogen-sensitive thin film beingcomprised of a sol-gel coating of nanostructured indium oxide doped tinoxide, with nanostructured BaCeO₃ and Pt and other metal-catalystoverlayers, arranged for exposure to the environment and exhibiting adetectable change of physical property when the hydrogen-sensitive thinfilm is exposed to hydrogen at room temperature;

exposing the hydrogen-sensitive thin film based MEMS device to theenvironment;

outputting said detectable change of physical property when the presenceof hydrogen in the environment is detected; and

exposing the MEMS based hydrogen sensor device to an ultra-violet lightsource as a mean for regenerating and decontaminating the MEMS device(self-cleaning).

It is yet another object of the present invention to provide a solidstate micro electro-mechanical systems (MEMS) hydrogen-selective sensorthat has no moving parts, has a response time on the order of seconds,operates with minimum power consumption, does not require frequentcalibration, has a large dynamic detection range, can operate at roomtemperature, and can be readily embodied as a hand-held portableinstrument.

The present invention relates in one aspect to a hydrogen sensorplatform, comprising a hydrogen-sensitive thin film sensor element on asilicon (Si) wafer substrate wherein the hydrogen sensor platformreceives a sol-gel coating of an indium oxide doped tin oxide thin filmover the sensor platform.

In another embodiment the MEMS substrate having a sol-gel coating of anindium oxide doped tin oxide thin film is exposed to an ultra violetlight source as a means for removing contaminants and regenerating(self-cleaning) the MEMS hydrogen-selective sensor device.

In embodiments the insulating oxide layer may be thermally grown on thesubstrate surface or deposited by other methods such as vapor depositionor sputtering.

Another aspect of the present invention relates to the patterning ofelectrodes onto the MEMS device using known techniques, for example,photolithography or wet chemical etching. In embodiments, the patterningof each electrode is carried out using positive or negative photoresist,or a known lift-off method.

In one embodiment, the hydrogen-sensitive, indium oxide doped tin oxidethin film is overlaid by a hydrogen-selective layer protecting the thinfilm hydrogen-sensor from deleterious interaction with non-hydrogencomponents of the environment being monitored, such as carbon monoxide,carbon dioxide, alcohols, hydrogen sulfide, ammonia, and hydrocarbonsetc. The hydrogen-selective layer may include nanostructured bariumcerate, or stronsium cerate or other proton conducting membranes and thenanoclusters of catalyst may include a metal such as Pt, Pd, Au, Ag orRh, and/or alloys thereof.

In another embodiment, the sol-gel derived nanostructured indium oxide(In₂O₃) doped tin oxide (SnO₂), with the sol-gel or microemulsionderived hydrogen-selective overlayer, is dip-coated over the hydrogensensor platform before being wire-bonded to a plastic or ceramicpackage.

Other objects and advantages of the present invention will be more fullyapparent from the ensuing disclosure, illustrations, and appendedclaims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a top view of one embodiment showing the mask design of thehydrogen micro sensor.

FIG. 1B is an exploded view of the same mask design showing theresistive temperature sensor of the hydrogen micro sensor.

FIG. 2A illustrates the silicon substrate used for fabricating thehydrogen sensor.

FIG. 2B illustrates the oxidation step used for fabricating the hydrogensensor.

FIG. 2C illustrates the metallization step used for fabricating thehydrogen sensor.

FIG. 2D illustrates photolithography/etching step used for fabricatingthe hydrogen sensor.

FIG. 2E illustrates the application and coating of Nano-materials ontothe substrate of the hydrogen sensor.

FIG. 2F illustrates the wire bonding fabrication step.

FIG. 3 illustrates XPS analysis of sol-gel sip-coated MEMS deviceshowing the presence of In, Sn, O, and Pt.

FIG. 4 illustrates multilayer structure of MEMS-based hydrogen sensor,which is an enlarged view of an encircled portion in FIG. 2E.

FIG. 5 illustrates giant-room temperature hydrogen sensitivity for theMEMS based sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the disclosed embodiment of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangement shown since theinvention is capable of other embodiments. Also, the terminology usedherein is for the purpose of description and not of limitation.

The present invention relates to a micro electro-mechanical systems(MEMS) hydrogen sensor which integrates a nano-structured indium oxidedoped tin oxide thin film hydrogen sensor element, made using a sol-geldip-coating process, with an ultra-violet light source as a means fordecontaminating and regenerating the MEMS based hydrogen sensor. Thehydrogen sensor of the invention is a MEMS device that may be adapted ina variety of apparatus embodiments to accommodate the objects of theinvention.

The MEMS device of the present invention may be fabricated in a numberof ways, for example, the MEMS device may be formed from a substrate.Typical substrates may comprise a silicon wafer or glass with an oxidelayer added for insulation.

In embodiments, the substrate of the present invention may, for example,comprise a silicon wafer having a thickness of from about 5 micrometers(μm) to about 5 inches as shown schematically in FIG. 2A.

In other embodiments, the thickness of the substrate may range fromabout 1 to about 5 inches. In a specific embodiment, the thickness ofthe substrate is from about 3 inches.

The substrate is oxidized to provide an insulating layer of silicondioxide, as illustrated schematically in FIG. 2B. In various embodimentsof the instant invention, the oxidizing layer varies in thickness fromabout 0.05 to about 2 micrometers (μm). In a specific embodiment, oxidelayer is about 0.5 micrometers (μm).

The substrate may be oxidized thermally in air at a temperature of fromabout 100 to about 600 degrees Celsius for from about 1 to about 3hours. In a specific embodiment, the substrate is oxidized thermally inair at from about 100 to about 200 degrees Celsius for 1 hour.

After photolighography or electron beam lithography, the electrode ispatterned using known methods such as, for example, wet or dry etchingas illustrated in FIG. 2D. Alternatively, a lift-off technique can beused, in which patterning is achieved by the dissolution of photoresistfollowed by deposition of a metallic layer of photolithographically ore-beam lithographically defined photoresist layer.

The physical gap between the electrodes varies in embodiments from about100 nanometers (nm) to about 100 micrometers (μm). In a specificembodiment, the gap varies from about 10 to about 50 μm.

In embodiments, a sol-gel process is used for coating the indium oxide(In₂O₃) doped tin oxide (SnO₂) as shown in FIG. 2E. The chemistry of thenanoparticles are listed in FIG. 3 (XPS spectrum). In addition to tinoxide, other oxides, such as, for example, titania (TiO₂), iron oxide(Fe₂O₃), and zinc oxide (ZnO) are suitable for embodiments of thepresent invention.

The sol-gel process is a versatile solution process for making ceramicand glass materials. In general, the sol-gel process involves thetransition of a system from a liquid “sol” (mostly colloidal) into asolid “gel” phase. Applying the sol-gel process, it is possible tofabricate ceramic or glass materials in a wide variety of forms:ultra-fine or spherical shaped powders, thin film coatings, ceramicfibers, microporous inorganic membranes, monolithic ceramics andglasses, or extremely porous aerogel materials. The starting materialsused in the preparation of the “sol” are usually inorganic metal saltsor metal organic compounds such as metal alkoxides. In a typical sol-gelprocess, the precursor is subjected to a series of hydrolysis andpolymerization reactions to form a colloidal suspension, or a “sol”.Further processing of the “sol” enables one to make ceramic materials indifferent forms. Thin films can be produced on a piece of substrate byspin-coating or dip-coating. When the “sol” is cast into a mold, a wet“gel” will form. With further drying and heat-treatment, the “gel” isconverted into dense ceramic or glass articles. If the liquid in a wet“gel” is removed under a supercritical condition, a highly porous andextremely low density material called “aerogel” is obtained. As theviscosity of a “sol” is adjusted into a proper viscosity range, ceramicfibers can be drawn from the “sol”. Ultra-fine and uniform ceramicpowders are formed by precipitation, spray pyrolysis, or emulsiontechniques.

In embodiments of the present invention a sol-gel coating comprising anindium oxide doped tin oxide thin film is applied over the hydrogensensor MEMS platform using known coating techniques such as, forexample, spin or dip coating. After the application of the nanomaterial(In₂O₃doped snO₂), the material is then dried at a temperature of fromabout 100 to about 200 degrees Celsius. Annealing is carried out attemperature of about 300 to about 1000 degrees Celsius after sputteringa thin film of Pt over the sensor-material.

Once the hydrogen-sensing layer has been formed, a hydrogenselectivelayer is deposited over the hydrogen-sensing layer. Thishydrogen-selective overlayer may be comprised of a barium cerate orstronsium cerate or other proton conducting membrane with the surfacemetal catalyst which may be of platinum, palladium, gold, silver,rhuthenium, and/or alloys thereof. In embodiments, thehydrogen-sensitive layer has a thickness of from about 100 to about 500nanometers. In one specific embodiment, the hydrogen-sensitive thin filmhaving thickness of 100 to 150 nanometer (nm) is deposited followed by ahydrogen-selective layer of barium cerate overlayer and Pt catalysthaving a thickness of from about 5 to about 50 nanometers, asillustrated schematically in FIG. 2E.

As a further variation, the hydrogen-selective over layer may be formedof nanostructured barium cerate, stronsium cerate or other protonconducting membrane or hydrogen permable membranes. For example, thethickness of the hydrogen-sensitive material thin films ranges fromabout 100 to about 500 nm thick, more specifically from about 100 toabout 150 nm thick, with a hydrogen-selective layer when present havinga thickness of from about 5 to about 50 nm, and more specifically fromabout 5 to about 20 nm. The hydrogen-selective over layer is, inembodiments, thick enough to adequately protect the sensor from othergases in the environment and thin enough to leave unchanged theproperties being monitored in the operation of the device.

The hydrogen-selective over-layer may be deposited or formed over thehydrogen-sensitive film in any suitable manner, including spraying,solution deposition (sol-gel and microemulsion and other techniques),dipping , chemical vapor deposition, physical vapor deposition, focusedion beam deposition, sputtering, etc. Generally, the methods describedherein for formation or coating of the hydrogen-sensitive thin film inthe first instance may also be used for forming the hydrogen-selectiveover-layer thereon, and vice versa.

The hydrogen-selective over-layer may be formed of any suitable materialof construction, which is suitably effective to prevent chemicalreaction or sorption processes from occurring that would preclude theefficacy of the hydrogen-sensing film for hydrogen sensing.

The selectivity exhibited by the proton conducting membrane filmsallows, for the first time, fabrication of inexpensive hydrogen sensorsthat can be deployed in large numbers to remotely monitor hydrogenlevels over large areas. Furthermore, hydrogen-selective films canoperate in an industrial or manufacturing environment containing traceorganic vapors.

The hydrogen sensing films can be coated with materials such asnanostructured barium cerate, stronsium cerate or other protonconducting membranes or hydrogen permeable membranes to provide aneffective barrier to the other gases in the environment, yet enable onlyhydrogen to diffuse through to the hydrogen-sensing thin film, therebyacting as a selective membrane for hydrogen in the sensor element.

The hydrogen-sensing thin film sensor element of such a hydrogen sensormay comprise a semiconductor thin film (i) arranged for exposure to anenvironment susceptible to the incursion or generation of hydrogen and(ii) exhibiting a detectable change of physical property when thehydrogen sensing film is exposed to hydrogen. Such detectable change ofphysical property may comprise optical transmissivity, electricalresistivity, electrical conductivity, electrical capacitance,magneto-resistance, photoconductivity, and/or any other detectableproperty change accompanying the exposure of the thin film sensorelement to hydrogen. The hydrogen sensor may further include a detectorconstructed and arranged to convert the detectable change of physicalproperty to a perceivable output, e.g., a visual output, auditoryoutput, tactile output, and/or auditory output.

The MEMS hydrogen sensor platform is then bonded using known techniquessuch as, for example, wire-bonding, ball-bonding, or flip-chip bondingas shown schematically in FIG. 2F and the cross section is shown in FIG.4.

Wire bonding is an electrical interconnection technique using thin wireand a combination of heat, pressure and/or ultrasonic energy. Wirebonding is a solid phase welding process, where the two metallicmaterials (wire and pad surface) are brought into intimate contact. Oncethe surfaces are in intimate contact, electron sharing or interdiffusionof atoms takes place, resulting in the formation of wire bond. In wirebonding process, bonding force can lead to material deformation,breaking up contamination layer and smoothing out surface asperity,which can be enhanced by the application of ultrasonic energy. Heat canaccelerate interatomic diffusion, thus the bond formation.

The Wire bonding process begins by firmly attaching the backside of achip to a chip carrier using either an organic conductive adhesive or asolder (Die Attach). The wires then are welded using a special bondingtool (capillary or wedge). Depending on bonding agent (heat andultrasonic energy), the bonding process can be defined to three majorprocesses: the microcompression bonding (T/C), ultrasonic bonding (U/S),and thermosonic bonding (T/S), as shown in Table A1.

TABLE A1 Three wire bonding processes. Ultrasonic Wire bonding PressureTemperature energy Wire Pad Thermo High 300-500° C. No Au, Al, Aucompression Ultrasonic Low 25° C. Yes Au, Al Al, Au Thermo sonic Low100-150° C. Yes Au Al, Au

The method of wire bonding that is most popular today is gold ballbonding, a process that melts a sphere of gold on a length of wire,bonds that down as a first bond, draws a loop out, and then connects thewire bond (the second wedge bond) down by means of a crescent and thenreforms another ball for the subsequent first ball bond.

Flip chip microelectronic assembly is the direct electrical connectionof face-down (hence, “flipped”) electronic components onto substrates,circuit boards, or carriers, by means of conductive bumps on the chipbond pads. In contrast, wire bonding, the older technology which flipchip is replacing, uses face-up chips with a wire connection to eachpad.

Flip chip components are predominantly semiconductor devices; however,components such as passive filters, detector arrays, and MEMS devicesare also beginning to be used in flip chip form. Flip chip is alsocalled Direct Chip Attach (DCA), a more descriptive term, since the chipis directly attached to the substrate, board, or carrier by theconductive bumps.

Eliminating packages and bond wires reduces the required board area byup to approximately 95%, and requires far less height. Weight can beless than approximately 5% of packaged device weight. Flip chip is thesimplest minimal package, smaller than Chip Scale Packages (CSP's)because it is chip size.

There are three stages in making flip chip assemblies: bumping the dieor wafer, attaching the bumped die to the board or substrate, and, inmost cases, filling the remaining space under the die with anelectrically non-conductive material. The conductive bump, theattachment materials, and the processes used differentiate the variouskinds of flip chip assemblies

In embodiments, a ultra-violet light source is assembled facing the MEMSbased hydrogen sensor. The UV light source is used for burning organiccontaminates from the MEMS device and as a light source. Thisdecontamination produces a clean sensor surface suitable for sensinghydrogen at room temperature. The UV source is turned off duringhydrogen sensing tests and the sensor detects hydrogen very efficientlyat room temperature. (FIG. 5 shows a giant room temperaturesensitivity).

The MEMS based hydrogen sensor device may be connected by a signaltransmission line to the central processor unit, which may comprisemicroprocessor or computer control elements for actuation, monitoringand control of the hydrogen sensor device. The central processor unitprocesses the signal carried by signal transmission line, and producesan output signal that is transmitted in signal transmission line to anoutput device, which produces an output that is indicative of thepresence or absence of hydrogen in the environment to which the sensoris exposed.

The output of the central processor unit may include any perceivableoutput, such as auditory output, visual output, tactile output (as forexample when the hydrogen sensor apparatus is adapted to be worn on thebody of a user, and the central processor unit comprises a vibratorimparting vibratory sensation to the user's body when hydrogen isdetected in the environment, such as may be useful in environments whereauditory or visual outputs are not readily perceivable.

In lieu of producing an output which is perceivable, the centralprocessor unit 44 may be programmed to actuate means for eliminatinghydrogen from the environment being monitored, as for example a sweepgas flushing operation to purge the environment of the hydrogen gas.

In embodiments, the MEMS base hydrogen senor operates in widetemperature of from about 15 degrees Celsius to about 650 degreesCelsius.

EXAMPLE 1 MEMS Based Sensor Platform Fabrication

A 3″ Si (100) wafer is used as a substrate for sensor fabrication. Ontop of the substrate 0.1 to 1 mm of silicon oxide is thermally grown.Alternatively, oxide can be deposited by other methods such as CVD orsputtering. Oxide is used as an insulation layer. Alternatively, glasssubstrate can be used. 10-50 nm-thick chromium (Cr) or titanium (Ti) and100-500 nm-thick gold (Au) films are deposited by thermal or e-beamevaporation on top of the oxide layer or on the glass substrate. Theinterdigitated electrodes were patterned on the substrate usingphotolithography and wet chemical etching. Positive or negativephotoresist was used for patterning the electrodes. Alternatively, alift-off method is used to pattern the electrodes. The gap betweenelectrodes is kept in the range of 10 nm to 50 mm. After sol-gel coatingof In₂O₃ doped-SnO₂ thin films over the sensor platform, in whichcoating process is outlined in the following section, the MEMS sensorplatform is wire-bonded to a plastic or ceramic package as illustratedin FIG. 2 schematically.

EXAMPLE 2

Tin(IV)-isopropoxide (Sn[OC₃H₇]₄) (10 w/v %) in iso-propanol (72 vol. %)and toluene (18 vol. %) and indium(III)-isopropoxide (In[OC₃H₇]₃) areprepared and mixed. Small glass substrates (1cm×1cm) are cut from thePyrex glass slides for the dip-coating experiments. The tin oxide (SnO₂)semiconductor thin film coating, in doped and undoped forms, is combinedon the Pyrex glass (silica) slides (substrate) and fabricated viasol-gel dip-coating technique. The glass substrates are ultrasonicallycleaned, first in acetone and then in iso-propanol. The pre-cleanedsubstrates are dipped in the solution of tin-isopropoxide iniso-propanol and toluene, having a concentration of 0.23 M oftin-isopropoxide, using a dip-coater with a withdrawal speed of 150cm/min. Indium(III)-isopropoxide is dissolved in this solution to obtainthin films of SnO₂ containing 6.5 mol % In₂O₃. The gel films formed aredried at a temperature of from about 150 to about 200 degrees Celsiusfor about 1 hour in air. The substrates are then dip-coated again usingthe same solution under similar conditions and dried again at from about150 to about 200 degrees Celsius for about 1 hour in air. A thin layerof Platinum is sputtered for about 10 sec on the dried thin films usinga sputter coater. The dried and Pt-sputtered gel films are then fired ata temperature of from about 400 to about 600 degrees Celsius in air. Thesamples are heated at a rate of about 30° C./min up to the firingtemperature, held at that temperature for about 1 h, and then cooled toroom temperature (20° C.) inside the furnace.

EXAMPLE 3

Fabrication of a MEMS Based Hydrogen Sensor

Micro structures were fabricated through a commercial foundry and theas-received die was micro machined using XeF₂ as a silicon selectiveetchant. A photolithographic lift-off process was used in combinationwith physical vapor deposition (PVD) to sequentially deposit agold/titanium thin film overlaid by a indium oxide doped tin oxide onthe suspended micro structures. The resulting devices were wire bondedand packaged in 40 pin ceramic chip carriers.

The fully packaged chips were placed in a sealed chamber, and electricalcontact made via feedthroughs into the chamber. Nitrogen and hydrogenwere introduced into the chamber and controlled with mass flowcontrollers and actuated valves. The resistance of the sensing film wasmeasured periodically with a digital multimeter and logged on a desktopcomputer.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

1. A hydrogen sensor device that detects hydrogen at room temperature,comprising: (a) a micro electro-mechanical system (MEMS) formed on asubstrate comprising: (i) a semiconductor substrate; (ii) an oxidizedlayer over said substrate; (iii) an interdigitated electrodeconfiguration formed over said oxidized layer; (b) a hydrogen sensitivesemiconductor oxide thin film over the interdigitated electrodes; (c) ahydrogen-selective membrane layer formed over the hydrogen-sensitivesemiconductor oxide thin film; and (d) a detector for detecting presenceof hydrogen from the hydrogen senser device by measuring a change of anelectrical property at room temperature; and (e) an ultra-violet sourcefocused on said hydrogen-selective membrane layer for regenerating anddecontaminating the hydrogen sensor device.
 2. The hydrogen sensordevice according to claim 1 wherein said semiconductor substrate iscomprised of silicon (Si).
 3. The hydrogen sensor device according toclaim 1 wherein said semiconductor substrate is comprised of glass. 4.The hydrogen sensor device of claim 1, wherein said hydrogen sensitivesemiconductor oxide thin film comprises nanostructured indium oxidedoped tin oxide.
 5. The hydrogen sensor device of claim 1, wherein saidhydrogen-selective membrane layer is selected from platinum, palladium,gold, silver, or rhuthenium.
 6. The hydrogen sensor device of claim 1,wherein said hydrogen sensitive semiconductor oxide thin film has athickness of from about 100 nm to about 500 nm.
 7. The hydrogen sensordevice of claim 1, wherein said hydrogen-selective membrane layer has athickness of from about 5 nm to about 50 nm.
 8. The hydrogen sensordevice of claim 1, wherein said hydrogen sensitive semiconductor oxidethin film is formed by a sol-gel process is comprised of a dopant in tinoxide.
 9. The hydrogen sensor device of claim 8, wherein said dopant iscomprised of about 5 to about 15 mole percent of indium-oxide.
 10. Thehydrogen sensor device of claim 9, wherein said dopant is comprised ofabout 6.5 mole percent of indium-oxide.
 11. The hydrogen sensor deviceof claim 1, wherein said interdigitated electrode configuration has anelectrode gap of about 20 micrometers.
 12. The hydrogen sensor device ofclaim 9, wherein said indium oxide doped tin-oxide hydrogen sensitivesemiconductor oxide thin film is deposited on the interdigitatedelectrode configuration.
 13. The hydrogen sensor device of claim 1,wherein the detector includes detecting a detectable change of aphysical property comprised of electrical resistivity or electricalconductivity of the hydrogen sensitive semiconductor oxide thin film.14. The hydrogen sensor device of claim 13, wherein the detectorincludes an output selected from the group consisting of visual outputs,auditory outputs, tactile outputs, and auditory outputs.
 15. Thehydrogen sensor device of claim 1, wherein said detectable change of theelectrical property occurs when the hydrogen sensitive semiconductoroxide thin film sensor is contacted with hydrogen gas.
 16. The hydrogensensor device of claim 1, further comprising a power supply for thehydrogen sensor device.
 17. The hydrogen sensor device of claim 1, wherein said device detects the hydrogen at room temperature and air-pressurewithin the range of about 50 to about 760 Torr, and with hydrogenselectivity over carbon monoxide, carbon dioxide, nitrogen monoxide,nitrogen dioxide, and hydrogen sulfide with a detection and recoverytime of about 50 to about 200 sec.
 18. The hydrogen sensor device ofclaim 17, wherein the hydrogen is detected up to about 900 ppm.